1. Field of the Invention
This invention relates to a die and a method of making the die. The die is adapted to stamp out from metal foil, circuit elements of a circuit board pattern and simultaneously bond the circuit elements to a substrate. More particularly this is a high precision die capable of forming circuit boards from relatively thick foil sheets, with the width dimensions of the circuit elements and the spacings thereof being of relatively small dimensions.
2. Description of the Prior Art
In my earlier patent, U.S. Pat. No. 3,758,350, entitled "Method of Making a Die for Stamping Out Circuit Boards", there is disclosed a method of etching a die block to form an improved die particularly adapted for forming a relatively compact circuit board where the circuit elements are of relatively small width and with small separation therebetween. In that method, first there is formed a conventional flat face die, comprising flat face die elements with intervening recess areas, this being accomplished in a conventional manner. Then the recess areas are filled with an acid resisting epoxy, and a photo resist is applied over the epoxy in a manner to overlap the outer edge portions of the die elements so that the middle portion of each die element is exposed. The die is then exposed to an etching medium to form a die element recess in the middle portion of each die element, with shoulders being formed along the edge portions of each die element. Next the acid resist material is removed, and with the recess filling epoxy still remaining the die surface is again etched for a short period of time. In one form, the final etching is done in a manner to provide "modified" die element edge portions where the inner edge surface of each die element edge portion is somewhat rounded, and in another embodiment, the final etching is accomplished in a manner to provide a knife edge die.
While the method described above has proven quite effective in making precision dies effective for high production manufacture of circuit boards, there is still a need for further improvement in the die making process. For many industrial applications it is desirable to have the elements of the circuit board made from a thicker foil sheet to enhance the reliability of the circuit elements being formed and their capability of handling higher current loads. Yet there is the continuing demand for yet greater compactness, which means making both the individual circuit elements and the separating areas between the elements of smaller width dimensions.
In a circuit board manufacturing operation, it is desirable to accomplish both the forming of the circuit elements from the foil sheet and the bonding of these circuit elements to a dielectric substrate in one stamping operation. This is usually accomplished by providing a thermo-adhesive coating between the foil sheet and the substrate and heating the die prior to the stamping operation. When the heated die is brought into engagement with the foil sheet, the die elements press into the foil sheet to separate the circuit elements from the foil sheet and press these into firm engagement with the substrate while heating the underlying adhesive layer to cause bonding of the circuit elements to the substrate. The excess foil adjacent the die elements does not become bonded to the substrate, and this excess foil is simply stripped away from the dielectric material after the die is moved away from the circuit board just formed.
However, when it is attempted to use heavier foils to make circuit boards where the width and spacing of the elements are still quite small, the problems of properly accomplishing this simultaneous forming and bonding of the circuit elements are greatly aggravated. First, with regard to the individual die element, the edge portions of each die element must be of a height sufficient to cause a separation through the entire thickness of the foil sheet. Secondly, the depth of the recess area of the die must be sufficient to permit the excess foil to become positioned in such recess area during the stamping operation, without the excess foil becoming bonded to the substrate. With regard to this problem of the unintended bonding of the excess foil, it is necessary to make a distinction between "background" areas of the die and "separating" areas of the die and of the circuit board being formed. The "separating" areas are those existing between two adjacent circuit elements, and with the type of circuit boards being considered herein, these separating areas are in the order of 15 to 50 thousandths of an inch in width. The "background" areas are those of substantially greater dimensions that separate groups of circuit elements being formed. Both the separating areas of the die and the background areas must be recessed to a depth sufficient to accommodate the excess foil. But in the background areas there is need for yet greater depth because of a tendency of the larger pieces of excess foil to buckle or bend during the stamping process and thus cause unintended bonding of this excess foil to the substrate during the stamping operation.
When it is attempted to employ etching techniques to achieve greater depth in the recess areas being formed, the problems in maintaining a high degree of precision, particularly with respect to adhering to a preselected pattern within relatively close tolerances, are substantially aggravated. When a resist pattern is applied to the surface of a die block and the etching solution is applied to the surface, the etching solution tends to work not only downwardly in the exposed area, but also laterally to "undercut" the material beneath the resist pattern. However, the degree of such undercutting is by no means always uniform. First, impurities or lack of uniformity in the material being etched (e.g., carbon particles in steel) can impede the progress of the etching medium in certain areas. Second, the resist material that overhangs the undercut may break away in certain areas, with the result that the undercut areas with the more exposure etch away at a faster rate than those areas where the existing overhang impedes access of the etching medium to the material just below the resist pattern. Thus, there is a tendency for the final configuration of the die elements to have "strayed" from the pattern of the original art work according to which the die is to be formed. These same problems exist in attempting to obtain greater depth in the recess areas in the faces of the die elements themselves, in that the undercutting of the etching medium causes greater inaccuracy in the final configuration of the upstanding edge portions of the die elements.
It can be appreciated that this problem becomes especially critical when it is attempted to form the edge portions of the individual die elements within quite close tolerances (e.g., a thousandth of an inch or less) so that these can properly perform their function of separating the quite small circuit elements from the foil, which may be as small as 10 thousandths of an inch wide. Not only is this precision of importance in the initial formation of the die, but also with regard to future maintenance of the die. As the edge portions of the individual die elements become worn, resharpening becomes necessary, and with the very small dimensions involved, this resharpening can best be accomplished by means of etching. Such resharpening can only be accomplished properly if the initial formation of the circuit elements is within proper tolerances.
In view of the foregoing, it is an object of the present invention to provide an improved die and a method of making a die within very close tolerances, which die is particularly adapted for forming a circuit board from relatively thick foil, with the width dimensions and spacing of the circuit elements being of a relatively small dimension.